CD4 is a 55 kilodalton transmembrane glycoprotein that exists in close association with p56-lck tyrosine kinase; cD4 is important in T cell activation, HIV susceptibility, and as the natural ligand for Class II major histocompatibility complex. The decline of cD4+ T cell number and function is believed to result in AIDS. Since severe cD4 cell functional defects both precede significant numerical loss of cD4+ T cells and occur when less than 0.1% of cD4+ T cells are infected, indirect viral mechanisms such as syncytium formation, autoimmunity, or immune system inhibition by viral products have been suggested to account for early functional defects. Recent estimates of increased viral burden later in disease are consistent with the logical hypothesis that HIV kills T cells in vivo, but the mechanism of direct viral killing of T cells is elusive. One current hypothesis suggests that inappropriate induction of programmed cell death (apoptosis) due to altered signaling through CD4/TCR may account for the qualitative and eventual quantitative defects in CD4+ T cells. A lack of syncytia in lck-negative or tyrosine kinase-inhibited cells has suggested that cD4-associated tyrosine kinase signaling is involved in syncytium formation as well. The crucial point is that, if signaling defects cause functional and numerical loss of CD4+ T cells, these putative aberrant signals (and the loss of cD4+ T cells in AIDS) may be amenable to therapy. Preliminary results from my laboratory indicate that T cells expressing altered CD4 possess altered susceptibility to HIV-induced programmed cell death and syncytia. This laboratory has isolated and defined a series of four types of clones made by transfecting cDNA encoding CD4 into a T cell tumor , HSB-2, that is CD4/CD3/TCR negative. These four clones are designated 1) GPiCD4 for a glycosylphosphatidylinositol (GPi)-linked CD4 clone, 2) HSBCD4 for a clone expressing low (comparable to other T cell lines) levels of wild type CD4 (wtCD4), 3) CD4/GPiCD4 dual expresser for an HSB T cell clone expressing both wtCD4 and GPi-linked CD4, and 4) HSB10XCD4 for a clone expressing ten times the amount of surface CD4 as HSBCD4. Using these cell lines I have observed four different outcomes of HIV infection: 1) in GPiCD4: infection, no syncytia, no apoptosis, and no cell death, 2) in HSBCD4: infection, syncytia, apoptosis and cell death, 3) in CD4/GPiCD4 dual expresser: infection, syncytia, and minimal apoptosis, and 4) in HSBl0XCD4: transient infection, no syncytia, and no observable cell death. Signaling involving the transmembrane region and cytoplasmic tail of CD4 is critical to the development of the CD4+ cell lineage. Similarly, the transmembrane and cytoplasmic region of CD4 is critical for HIV cytopathic effect in the HSB-2 T cell transfectants. The high CD4 expressor, HSB10XCD4, only transiently produced HIV, suggesting either that transient HIV infection was due to death of HIV+ HSB l0XCD4 cells before the spread of infection or that surface CD4 influences viral infectivity. The HSB transfectants will allow me to conduct precise studies of I) cellular resistance to HIV cytopathic effect in GPiCD4 and 2) cellular resistance to HIV infection in HSB10XCD4. These studies will focus on the roles of CD4, associated cellular signalling molecules, and HIV envelope proteins in the pathogenesis of HIV.